"One of the strangest and least-studied enigmas, though, is how tattoos survive at all. Our immune system is constantly doing its darndest to destroy them—and understanding why it fails could clue us in to one of our bodies’ most important functions, even when we leave the skin blank."

https://www.theatlantic.com/science/archive/2023/03/tattoo-science-immune-system-effects/673462/